This invention relates generally to the plating of tubing and other shapes. In particular, the present invention relates to an apparatus and method for continuous galvanizing of tubing.
The prior art has traditionally utilized a molten metal bath such as zinc contained in a reservoir located under the plating trough. The molten zinc is pumped into a separate trough located at the product passline. The pumping rate is adjusted to maintain galvanizing temperatures in the trough of between 820.degree. F. to 850.degree. F. The primary disadvantage with this prior art method is that the initial design capacity is limited to a predetermined throughput; if the workpiece preheat levels are insufficient or if the pump rates are insufficient, the bath temperature in the trough will drop below minimum galvanizing temperatures because of the considerable amount of heat given up from the bath to the lower temperature tubing or other shape.
A further disadvantage of this prior art method is that rather large amounts of molten zinc are recirculated from the plating trough to the primary reservoir of molten zinc. This relatively large return of molten zinc from the plating trough increases the requirement for heat input to and higher pumping rates for the lower reservoir of molten zinc. Additionally, this practice tends to return molten zinc to the lower or primary reservoir which has begun to oxidize. The higher pump rate for the reservoir causes turbulence which in turn creates significant dross formation and kettle wash out or erosion.